I have developed my own theory about this because none included in the link above were very convincing. First of all, it's well established that animals evolve quickest to secure mating. For instance animals will breed themselves into extinction or will make it more difficult to survive for the right to mate and examples of this are peacocks - male birds evolved their elaborate tails because peahens (girl peacocks) like them. So the guy bird displays his lovely tail and the girl checks it out and if she is impressed she'll select him to mate with. The problem with the tail is that it is heavy and huge and makes it more difficult for the male bird to escape a predator.
The case of the Irish Elk, the female elk's selected for males with the largest antlers to the point where the antlers became so large and heavy that it was difficult for the males to navigate forests, and even being able to lift his head. The species breed themselves into extinction.
This is what I think is going on with the female breasts. We evolved them despite how bad they are for human females - they are more likely to get cancer than other female mammals' mammary glands, they can be bad for the human's posture and make it difficult to run thus impinge on her ability to survive.
Nevertheless, men chose to breed with women with larger breasts and those were the women men helped raised their offspring.
I once read this comment on a discussion about abortion, that if you are a woman and enter 'wet t shirt' kinds of events then it's better to get pregnant before the event because it enlarges your breasts... and afterwards you can get an abortion. While I do not approve of either wet t shirt competitions and I definitely don't approve of getting pregnant to get bigger breasts and definitely don't think that abortion is a good solution, I also think that such a woman would NOT be a desirable mother and so abortion is probably the best outcome.
Anyway, this got me thinking that if guys are attracted to bigger boobs then evolving them is definitely a good idea especially if you want to attract a man for the purposes of breeding. Why are men attracted to boobs in the first place? When we have lost most markers that indicate a woman is 'in season', besides we don't want to merely attract a mate to merely breed, we need him to stick around until we give birth and afterwards so he can help raise the child and to bring home a share of whatever to help us survive whether that was originally meat, and now, money.... indicating breeding, boobs are a good indicator of fertility - she's already pregnant - hence she has boobs.
I think men feel protective of women who are pregnant.
Of course male humans behave like peacocks themselves, by flashing how much money they have around. Men making money is important in attracting a mate these days. Again it is upto women to choose their men wisely - we need to choose kind, good men rather than the kind of men who will do what it takes to make money.